Conventionally, turbochargers that turbocharge an internal combustion engine using exhaust gas pressure have been widely used. In common turbochargers, a compressor for compressing incoming air and transferring the compressed air into cylinders is provided in an intake passage, and a turbine that rotates upon receiving exhaust gas pressure is provided in an exhaust passage. A rotary shaft of the turbine is connected to a rotary shaft of the compressor, and rotative force of the turbine is used as drive force for the compressor. By increasing the amount of air using exhaust gas pressure, the output of an internal combustion engine can be increased without compromising fuel economy performance.
Techniques for adjusting turbocharging pressure by controlling the RPM of the turbocharger are known. For example, as described in Japanese Laid-Open Patent Publication (Kokai) No. H09-144575, there has been the technique that a bypass passage connects an exhaust passage upstream of the turbine and an exhaust passage downstream of the turbine together, and the path over which exhaust gas is circulated is changed according to the operating state of the internal combustion engine so as to increase or decrease turbocharging pressure.
According to this technique, a wastegate valve whose degree of opening is changeable is disposed on the bypass passage, and the degree of opening of the wastegate valve is controlled according to turbocharging pressure in the intake passage. When the degree of opening of the wastegate valve is increased, the flow rate of exhaust gas detoured around the turbine and discharged from the bypass passage increases, which causes the RPM of the turbine to decrease. In response to this, turbocharging pressure generated by the compressor also decreases. When the degree of opening of the wastegate valve is decreased, the amount of exhaust gas introduced into the turbine increases, which causes the RPM of the turbine to increase and causes turbocharging pressure to increase, too. Thus, the RPM of the turbine can be changed by controlling the degree of opening of the wastegate valve, and even if the flow rate and pressure of exhaust gas from the internal combustion engine are constant, turbocharging pressure can be changed.
Also, as described in Japanese Laid-Open Patent Publication (Kokai) No. H2008-75549, there has also been developed the technique that turbocharging pressure is controlled to increase not only by controlling turbocharging pressure using the wastegate valve but also by assisting driving force of the compressor using an electric motor. The electric motor is disposed on, for example, a rotary shaft connecting the compressor and the turbine together, and configured to transmit both drive force from the turbine and drive force from the electric motor to the compressor. While the RPM of the turbine depends on the circulating state of exhaust gas, the RPM of the electric motor does not depend on the circulating state of exhaust gas. Thus, where drive force from the turbine runs short, stable turbocharging pressure can be ensured by driving the electric motor.